I'm fairly sure that this is not possible to enforce via Volksentscheid. Its state legislation, so the details differ, but as far as I understand in most states the referendum can only decide over a proposed law. In some states a general guideline for the parliament can be decided over (Berlin, HH and Schleswig-Holstein) but those are only recommendations.
I'm not certain if it would be a valid course to create a law that forces the parliament (or rather the states representatives in the Bundesrat) to propose a law. That might create a legal problem: What if the proposed law gets changed or rejected in the Bundesrat etc. You're free to try, but I doubt that course of action will work.
